How can I persuade my brother to give up his idea of buying another phone (details inside)

OK so it may sounds so silly, but the situation has got to the point where I can’t deal with it alone and have to ask for the collective’s help. My brother already owns two phones, one is an old-fashioned one strictly for calls, and the other is for all of his other stuff. Recently he wants to buy a new phone to shoot video of himself to upload on Youtube. He’s a fanboy of Pewdiepie and he wants to be an established Youtuber just like him. He said he has already saved enough of his allowance and all he needs is someone to go out and buy it discreetly for him. I (and some of my friends who I asked for advice before) can see a lot of reasons why it’s a waste of money to buy another phones, the most obvious ones are that he already has two fully functional phones, and that if I agree to help him, it will teach him that he can be materialistic and needn’t value what he have as long as he has money.

The thing is, I already told him everything I said above, but all went through deaf ears. He could always find reasons to back up anything I said. I told him the other phone could record videos just fine, he would point out everything wrong with recording videos with that phone. I told him he is throwing away a fully functional phone just because it lacks only one function, he would say he will either use three phones or give me the old phone. I told him he is being wasteful, he would say he is just being serious about his work. I told him he should wait until he gets older and gathers more professional recording tools, he would say his dream can’t wait. I just can’t win the argument and right now he is pestering me, asking me when I will go buy the phone every four hours. He won’t leave me alone until he sees that phone right in my hand.

This is really getting on my nerve. I can’t go to my parents to settle this down. I know he trust me and if there is anything he can’t say to them, he will come to me. If I go to them he will get the wrong impression that I’m not to be trusted and I don’t want to lose his trust over something so trivial like this. But I just can’t convince him that his wish is unreasonable. And I just want him to stop harassing me! What can I do to resolve this mess?

If it is of any help, my brother is 13 years old, and is generally what you expect from an average 13-year-old.